A continuously variable transmission for a vehicle is an alternative to a standard planetary automatic transmission which is common throughout the world. The job of a transmission, in a vehicle is to change the speed ratio between the engine and the wheels of a vehicle. The transmission uses a range of gears from low to high to make more effective use of an engine's torque as the driving conditions change. Those gears can be engaged manually or automatically. In a traditional automatic transmission, the gears are interlocking toothed wheels that transmit and modify rotary motion and torque. A combination of planetary gears create the different ratios.
A continuously variable transmission does not have a gear box with a set of gears or interlocking toothed wheels. Most common continuous variable transmissions include a belt and an input pulley and an output pulley. As the pulleys are at variable diameters, the belt changes location creating the gearing effect. The drive or input pulley is connected to the crank shaft of the engine and the driven or output pulley transfers the energy to the drive shaft. As the two pulleys change their radii relatively to one another they create an infinite number of gear ratios.
The advantages of a continuously variable transmission of a standard planetary automatic transmission are that the continuously variable transmission creates a smoother ride in a vehicle because it eliminates the jolting of changing gears, improves fuel efficiency, eliminates gear hunting as the car decelerates, provides better acceleration, provides better control over emissions, and replaces inefficient fluid torque converters.
In an attempt to utilise continuously variable transmissions more effectively, a number of different types have been developed. For example, the toroidal continuously variable transmission replaces the belts and pulleys with disks and power rollers. Hydrostatic continuously variable transmissions work by varying the radius of the contact point between two rotating objects. In this type of transmission the rotational motion of the engine operates a hydrostatic pump on the driving side. The pump converts rotational motion in fluid flow. Then with a hydrostatic motor located on the driven side the fluid is converted back into rotational motion. When a hydrostatic transmission is combined with a planetary gear set and clutches a hybrid known as a hydromechanical transmission is created. At a low speed, power is transmitted hydraulically, and at a high speed, power is transmitted mechanically. Between these extremes the transmission uses both hydraulic and mechanical means to transfer power.
It has however been found that existing hydromechanical transmissions have a limited number of gearing ratios, are not fully controlled, cannot provide a reverse function, and require hydraulic boosting to start.